Less than 24 hours after he criticised Britain over the Falklands during a visit to Buenos Aires to see the Argentine president Cristina Fernández de Kirchner, Penn accused the UK of “insensitivity” for posting the heir to the throne to the disputed south Atlantic territory.

Penn accused the UK government of geopolitical grandstanding after a meeting with Uruguayan president Jose Mujica in Montevideo, a visit undertaken as part of Penn’s role as ambassador at large for Haiti. The presence of Prince William in the Falklands was an act of provocation, he suggested.

“It’s unthinkable that the United Kingdom can make a conscious decision to deploy a prince within the military to the Malvinas, knowing the great emotional sensitivity both of mothers and fathers in the United Kingdom and in Argentina who lost sons and daughters in a war of islands with a population of so few,” he said.

“There are many places to deploy the prince. It’s not necessary, when the deployment of a prince is generally accompanied by warships, to send them into the seas of such shared blood.”

The double-Oscar winner condemned criticism of his earlier comments as “hyperbole”. He said: “My oh my, aren’t people sensitive to the word colonialism, particularly those who implement colonialism.”

The leftwing actor, who has previously called for the impeachment of George W Bush over the Iraq war, praised America’s longstanding alliance with the UK but said he felt criticism was necessary. He added that he understood and respected the wish of people living in the Falkland Islands to remain British, but said Argentina and Britain needed to negotiate the sharing of the islands’ natural resources.

Tory MP and former army officer Patrick Mercer told the Daily Mail his comments were “moronic”. He said: “What on earth has this got to do with Sean Penn? He’s neither British nor Argentine and seems to know nothing about the situation judging by this moronic comment. A good number of his movies have been turkeys, so I suppose we shouldn’t expect much better coming out of his mouth.”

After an earlier meeting with the Argentinian president, Penn urged Britain to join UN-sponsored talks over what he called “the Malvinas Islands of Argentina”. He said: “It’s necessary that these diplomatic talks happen between the United Kingdom and Argentina. I think that the world today is not going to tolerate any kind of ludicrous and archaic commitment to colonialist ideology.”

Tensions have been growing between Buenos Aires and London for months, as the anniversary of the start of the Falklands war approaches on 2 April. British officials last week dismissed claims it was “militarising” the situation in the south Atlantic by deploying nuclear weapons nearby.

Argentina said it had intelligence that a Vanguard submarine had been sent to the area, and demanded to know whether it was carrying warheads. Britain insisted the deployment of one of its most modern destroyers, HMS Dauntless, to the region was merely routine.

The arrival of Prince William in the Falklands for a posting as an RAF search and rescue pilot has further infuriated Buenos Aires. And there were protests on the streets of the Argentinian capital after the website of Falklands newspaper the Penguin News ran a photo of Fernández labelled “bitch”.